Since polyamides (nylon) are generally excellent in mechanical properties and durability, they are used in large amounts as fibers for industrial materials such as tire cords, fibers for carpet and molded articles. On the other hand, polyamides are also used in large amounts as fibers for clothing, particularly for inner wear due to their excellent flexibility and dyeability.
Polyhexamethylene adipamide (nylon 66) is a typical representative of polyamides, and is also used as fibers for industrial materials, fibers for carpet, fibers for clothing, molded articles, and the like. Polyhexamethylene adipamide, however, has a serious disadvantage as fiber for clothing in that it suffers considerable thermal yellowing compared with poly-.epsilon.-capramide (nylon 6) which is also a typical representative of polyamides. The disadvantage matters a great deal regardless of whether it is used as fibers or molded articles, in application where the whiteness in external appearance is required. In addition to the fact that fibers for clothing are particularly susceptible to oxidation inherently due to the small single fiber diameter (large specific surface area), they are thermally set (heat treated) without exception in the process for producing fabrics and in fabrication. As a result, the problem of yellowing mentioned above is manifested. The use of polyhexamethylene adipamide in fibers for clothing is, therefore, extremely limited.
For example, inner wear such as lingerie and foundation garments is an appropriate application for polyamide fibers having flexibility and suitable hygroscopicity. Since polyhexamethylene adipamide fibers suffer considerable yellowing during thermal setting as described above, the whiteness of the products is lowered or the color development thereof becomes a dull shade (dull in shade). Accordingly, the polyhexamethylene adipamide fibers are not substantially used at present for inner wear. Poly-.epsilon.-capramide fibers and polyester fibers have overwhelmingly high shares at present in the application thereof to the inner wear.
Since the poly-.epsilon.-capramide fibers are significantly excellent in thermal yellowing resistance and suffer thermal setting-caused yellowing very slightly, compared with the polyhexamethylene adipamide fibers, they can be used for the inner wear substantially without a problem. Moreover, the polyester fibers have still more excellent thermal yellowing resistance than the poly-.epsilon.-capramide fibers, and suffer almost no yellowing caused by thermal setting, that is, the whiteness of the fibers is maintained.
Improving the thermal yellowing resistance of the polyhexamethylene adipamide fibers to a level comparable to that of the poly-.epsilon.-capramide fibers is, therefore, the first goal for the manufacturers thereof. Improving the resistance to a level comparable to that of the polyester fibers is the final goal for them. The techniques for achieving these goals, however, have not been created.
In melt molding and melt spinning thermoplastic organic polymer, a thermal stabilizer such as a thermal oxidation inhibitor is incorporated in the polymer starting materials to inhibit or decrease the thermal alteration such as thermal degradation and discoloration of the polymer. Although the selection of suitable additives depends on the types of polymers, examples the thermal stabilizer commonly used for polymers which have currently been used at large are phenolic antioxidants including hindered phenols. The level of degradation resistance of a polymer considerably differs depending on the requirement of polymer molded articles to be produced therefrom. As a result, various incorporation formulations such as the use of phenolic antioxidants in combination with other chemical substances, for example, organic antimony compounds, phosphorous compounds and thioether antioxidants have been attempted.
Japanese Patent KOKAI Publication No. 46-7455 discloses that the incorporation of organic antimony compounds and phenolic antioxidants prevents the oxidation deterioration of polymers of all types including polyolefin. Numerous phenolic antioxidants including 2,4-bis(alkylthio)-6-(3,5-dialkyl-4-hydroxyanilino)-1,3,5-triazine are exemplified in the specification of the patent publication. Concrete examples in which the triazine derivatives are applied to polypropylene as antioxidants are disclosed therein. The prior technique evaluates the antioxidation effect on the polymer by an oxygen absorption amount, and does not refer to the thermal yellowing prevention effect on the polymer.
Japanese Patent KOKAI Publication No. 6-16929 discloses that the incorporation of the three types of agents: hindered phenolic antioxidants, phosphorus type antioxidants and thioether type antioxidants, in polyamides can suppress the discoloration caused by oxidation deterioration of injection molded polyamide articles. The patent publication, however, discloses no quantitative effect of inhibiting discoloration. As a result of investigation, the present inventors have found that no satisfactory thermal yellowing resistance can be obtained even when the compounds in the patent publication are incorporated in the polyhexamethylene adipamide fibers. For example, when the fibers to be used for lingerie are to be dyed in pale pink or pale blue, the, desired color development cannot be obtained due to yellowing caused by thermal setting, and only products giving an impression of a dull shade can be obtained. That is, a thermal yellowing resistance which promotes the utilization of the polyhexamethylene adipamide fibers in the field of inner wear has not been obtained at any by the techniques disclosed in the patent publication. Moreover, among the antioxidants exemplified in the patent publication, there is no 2,4-bis(alkylthio)-6-(3,5-dialkyl-4-hydroxyanilino)-1,3,5-triazine.
The specification of Japanese Patent KOKAI Publication No. 51-1557 discloses that the incorporation of a phenolic antioxidant which completely differs from the compound of the present invention and which has a specific structure and, if necessary, a phosphorus compound can suppress the thermal yellowing of the polyamides. The patent publication, however, gives no description related to the effect of preventing thermal yellowing. As a result of investigation, the present inventors have found that sufficient inhibition of the thermal yellowing cannot be achieved even when the compounds disclosed in the patent publication are incorporated in the polyhexamethylene adipamide fibers. For example, when the fibers to be used for lingerie are to be dyed to pale pink or pale blue, the desired color development cannot be obtained due to yellowing caused by thermal setting, and only products giving an impression of a dull shade can be obtained. That is, a thermal yellowing resistance which promotes the utilization of the polyhexamethylene adipamide fibers in the field of inner wear has not been obtained at all by the technique disclosed in the patent publication.
Furthermore, since the compounds concretely disclosed in the specification of the patent publication have low heat resistances, adverse effects of deteriorating the compounds themselves have been caused when the compounds are melt kneaded with polyhexamethylene adipamide having a melting point as high as more than 260.degree. C.
The specification of U.S. Pat. No. 3,594,448 discloses that many types of hindered phenol compounds are effective in improving the whiteness of fibers obtained from a blend of polyamides and polyesters. Among the hindered phenol compounds, the triazine derivative I! mentioned above is also included. In the invention disclosed in the specification of U.S. Patent, however, a deterioration reaction at the boundary between the two components, the polyamide and the polyester, which reaction is a phenomenon specific to the blend thereof, is inhibited. As a result, yellowing which occurs after blending is improved.
Japanese Patent KOKOKU Publication No. 55-20498 discloses that the incorporation of three types of substances: a metal salt of boric acid, an organic phosphorus compound and a phenol having steric hindrance, in a polyamide having ring structures in the principal-chain can inhibit coloring and yellowing of the polyamide. The invention of the patent publication, however, solves the problem specific to a polyamide having ring structures. That is, the invention aims at preventing the polymer from lowering its molecular weight, gelling and yellowing by inhibiting the thermal decomposition reaction of the molecular chain thereof caused by the amino end groups thereof.
The specification of Japanese Patent KOKAI Publication No. 54-82496 discloses that 2,4-bis(alkylthio)-6-(3,5-dialkyl-4-hydroxyanilino)-1,3,5-triazine is effective in improving the thermal stability of a double bond-containing polymer such as rubber. The specification of the patent publication does not suggest that the compound is effective in inhibiting the yellowing of a polyamide, particularly the polyhexamethylene adipamide fibers.
It has heretofore been a general practice to inhibit the thermal deterioration of polyamides by incorporating N,N'-hexamethylenebis(3,5-di-tert-butyl-4-hydroxyhydrocinnamamide) (e.g., "Stabilization Formulations for Recent Development of the Application of Polyamide Resins," Ciba Geigy (Japan) Limited). However, even though the improvement of the mechanical thermal stability of the polyhexamethylene adipamide is achieved by the use of the compound, the effect of inhibiting considerable thermal yellowing specific to the polymer is extremely insufficient.
The amino end group concentration of the polyamide fibers is closely related to the deep dyeability of the fibers and the spinnability of the polyamide polymer. Polyamide fibers which have a high amino end group concentration and which are dyeable with an acid dye are commercially well known.
The specification of U.S. Pat. No. 3,078,248 discloses that when the amino end group concentration of polyhexamethylene adipamide is increased, drips frequently occur during melt spinning, and consequently normal spinning becomes difficult. Those skilled in the art know well that when the amino end group concentration becomes high, drips and breaks frequently occur, and that the frequent occurrence thereof is caused by the promotion of the thermal decomposition and branch-forming reaction of the amino end groups.
Accordingly, industrial spinning of the polyhexamethylene adipamide fibers has been achieved to a normal spinning level by the use of a polymer poor in the amino end group concentration, namely rich in the carboxyl end group concentration (e.g., Japanese Patent KOKOKU Publication No. 3-57966).
In general, organic phosphorus compounds having specific structures (specification of U.S. Pat. No. 3,078,248), and alkali metal salts of dicarboxylic acids or aminocarboxylic acids (Japanese Patent KOKAI Publication No. 1-104654) are disclosed as stabilizers for inhibiting drips and breaks during melt spinning (thermal decomposition inhibitors, branch-formation inhibitors).
An object of the present invention in the broadest sense is to raise the level of the properties of the polyhexamethylene adipamide fibers during use, particularly that of the spectrum of properties of fibers for clothing to the level comparable to that of the polycapramide fibers and the polyester fibers.
A concrete object of the present invention is to provide polyhexamethylene adipamide fibers having a thermal yellowing resistance at least comparable to that of the polycapramide fibers.
Another concrete object of the present invention is to provide polyhexamethylene adipamide fibers containing amino end groups of high concentration and having improved properties as fibers for clothing and an improved process for producing the same.
Still another concrete object of the present invention is to provide polyhexamethylene adipamide fibers having both thermal yellowing resistance to a high level and deep dyeability.
An intended object of the present invention is to provide a composition useful for obtaining molded articles such as polyhexamethylene adipamide fibers having thermal yellowing resistance.